1. Field of the Invention
The present invention relates to well logging systems and methods and, more particularly, to pulsed neutron well logging systems and methods.
2. Description of the Prior Art
Heretofore in a pulsed neutron logging system where a combination synchronization/stabilization pulse, hereinafter referred to as a sync/stab pulse, is generated before the neutron burst. The sync/stab pulse is used at the surface to trigger a window (gate) which occurs after the neutron burst. The detected gamma rays are only measured and counted during this window.
Two problems are created at the surface. The first problem is that the counts due to the sync/stab pulse will not be displayed in the spectrum since the gate is not open at the time the sync/stab pulse occurs. A second problem is that a single channel analyzer used to detect sync/stab pulses misses a sync pulse, due to "pile-up", noise, etc. It may be triggered by a pulse during the neutron burst. The erroneous triggering causes the time gate to open at the wrong time interval. The resultant shift in timing will give erroneous results and possibly allow some of the next burst through the gate. Since the gate is misplaced in time the next sync pulse may be missed and repeat the process until the synchronization circuit misses a full cycle while collecting data at the wrong times.
The present invention provides for the gate to be open when a sync/stab pulse occurs, thus allowing the recording of the sync/stab pulses. The present invention also closes the gate if the next sync/stab pulse does not occur during the narrow gate period used in the previous application to allow the sync/stab pulse through the gate. Since the narrow gate is generated by the previous sync/stab pulse, the main gate will open only if the present detected sync/stab pulse occurs during the small window. The probability of two noise pulses or pile-up pulses occurring at the right time in succession is very small. The blank or missing gates will have less effect on the accuracy of the data than the gates that are misplaced in time.